Thursday, July 30, 2015

Return of Monsoon Moisture and what is Dry Lightning?

Hey all! Looks like we are transitioning into a more active thunderstorm pattern once again. High pressure over the desert southwest will bring above normal temperatures for the next few days. Increasing moisture and instability will bring chances for thunderstorms Friday through Monday, mainly for the Sierra and for far western Nevada. Gusty outflow winds and lightning fire starts are the main concerns for Friday and Saturday with locally heavy rain possible by the weekend.

Just to be clear though, this monsoon push will not be like the one we experienced earlier in July. Although there could be some localized flooding we are not expecting multiple areas of flash flooding. Why? 

The moisture associated with this southeasterly flow will not have remnants of a tropical storm with it (Dolores in early July) and moisture doesn't appear to stick around as long as last time. It looks like southwesterly flow should return by sometime next week which will really dry out the region and limit showers and thunderstorms from continuing.

Showing the current visible satellite imagery from this morning, the monsoonal moisture is slowly making its way northward through California and southern Nevada. Notice the lightning strikes even this morning at 11:30AM. The graphic above shows that increasing chances for thunderstorms through the weekend and into early next week. Those with outdoor plans and activities should be sure to have a contingency plan for the possibility of lightning, gusty winds, and heavy rain by early next week. For the most up-to-date forecast please go here

So let's talk about lightning. Sometimes you will hear (or read) the forecasters here at NWS Reno refer to lightning or storms as 'dry lightning' or 'dry thunderstorms'. So what does that mean? In simplest terms it just means that lightning strikes are occurring without significant precipitation. So it isn't actually a reference to the type of lightning, but the type of storm that the lightning is associated with. The National Weather Service classifies a dry thunderstorm as a storm that produces less than a tenth of precipitation over an area. These types of storms are typical during the summer in the eastern Sierra and western Nevada, especially during the first couple days of a monsoon push. Dry lightning or dry thunderstorms are of great concern to the fire community as lightning with little to no rain can result in fire starts. Here is an example of a forecast sounding that would indicate the potential for dry lightning. If you need a refresher on looking at a Skew-t sounding go here.

There are a couple of things to note in this sounding. Precipitable water values with this sounding (not shown) are at 0.61 inches, which will likely be enough to produce thunderstorms, but right around average for this time of year. The main components of the sounding that would leave us to think of dry thunderstorms are the dry sub-cloud layer and the relatively fast winds in the steering layer. The more extensive the sub-cloud layer, the more difficult it is for the rain to moisten it and reach the ground, which commonly shows up as virga and/or dry microbursts. Also fast moving storms don't have as much time to put down rain on a particular point, so storms end up being on the dry side.

For the upcoming forecast we are expecting a mix of wet and dry thunderstorms, so while there will be some dry lightning we are not expecting a widespread lightning event that will start multiple fires. Remember that no where is safe outdoors when there is lightning. Lightning fatalities are up this year, so don't become a statistic! For more information on lightning safety, take some time to check out 

Official forecast discussion here

Sunday, July 5, 2015

Tools of the Trade: WSR-88D aka Doppler Radar

Welcome back! Glad that we haven't lost your interest ;-) It has been an incredibly busy couple of weeks with all of the thunderstorms across northeast California and western Nevada. Since it is thunderstorm season we thought this would be a great topic to discuss! Today we are going to focus on one of our most important tools of the trade, the doppler radar, aka WSR-88D (Weather Surveillance Radar 1988 Doppler).  Radar was invented during WWII and many surplus radars were given to the Weather Bureau (now the National Weather Service) after the war. WSR-57 and WSR-74 were the first radars built specifically for weather detection. WSR-88D was the first radar that could detect particle motion and they were deployed across the country in the mid to late 1990s. The current WSR-88D network includes 160 radars across the country as well as Alaska, Hawaii Puerto Rico and Guam (not pictured).

Radar is an acronym that stands for RAdio Detection And Ranging, but how does it work?

    1. Radar transmits a burst of radio waves.
    2. These waves bounce off of objects, like raindrops, snow, dust, birds and some of the energy is reflected back to the radar's receiver.
    3. The amount of radio wave energy scattered back to the radar determines the object's reflectivity.
Reflectivity is a function of: 
  • Size (radar cross section)
  • Shape (round, flat, etc.)
  • State (liquid, mixed, dry wet, etc)
  • Concentration (# of particles in a volume)
All of these characteristics of reflectivity help us to determine what the radar waves are hitting when they are transmitted.  Is it hitting rain, snow, dust, birds or bugs? The latest addition to the radar has been the upgrade to dual-polarimetric (Dual-Pol) technology that transmits radio waves oriented in the horizontal and vertical direction. NWS radars upgraded to dual-pol between 2011-2013. The dual-pol technology allows us to sample the size and shape of objects. This allows us to gather more details to improve estimates of precipitation type, intensity, as well as highlighting the difference between precipitation and non-precipitation echoes.

We are going to focus on a couple of the products we produce from the radar data and how we use them. Let's start with base reflectivity. Taken from the lowest elevation scan, base reflectivity is excellent for examining the region around the radar to look for precipitation. However, the radar beam increases in elevation as you get further away from the radar (shown below).

This is a problem that we deal with at NWS Reno since our radar is located on the top of Virginia Peak (elevation 8,370 feet) as noted by the fancy red arrow on the map below. Something about how the radar can't see what is going near the ground, and plug how spotters, facebook reports, yadda yadda helps us out.  Base reflectivity gives us a basic idea of the type of objects in the air, but we do need to use other products to get a more exact determination. Here is an example of base reflectivity from the NWS Reno Radar from June 10th, 2015.  The light green shades are typically rain or snow, and the higher the reflectivity values (into the yellows and reds) then we will usually pay more attention to those areas. Notice in the areas of red and yellow southwest of Lovelock and moving along Interstate 80. Those storms were putting down some pretty heavy rain and maybe even some small hail right across the interstate. Depending on the existing atmospheric environment, these storms can also put out some pretty hefty winds with gusts around 50 to 60 mph! To say the least, if we are seeing reds and yellows on the base reflectivity, we are definitely starting to pay more attention to those areas then the light green shades.  
Next product that we will discuss is base velocity, which displays the average wind speed of particles that are detected by the radar. The radar can measure the speed due to the Doppler effect, in which the frequency of a wave changes as it bounces off a particle that is moving with respect to an object that is not moving (the radar). Shown below is an example of base velocity that illustrates an EF2 tornado in Baker County Florida on April 15, 2007. Green colors indicate velocities towards the radar while red indicates velocities moving away from the radar. As you can see in the animation, the most interesting areas are where red and green colors meet, especially when the values are large! 

That wraps up our quick talk about just a couple of the radar products for now, but keep an eye out for future posts, especially about some of the dual-pol products! Check out the short video below that gives a tour of the NWS Reno radar on top of Virginia Peak. 

There are thunderstorms in the forecast for the rest of the week so remember to read the forecast before you head out on your adventures for the day! Forecasts can be found at

As always if you have any questions feel free to email